100 to 1 it's the sender.
Bit of a sod to get to but you can do it through the inspection hatch in the inner wing offside front wheel. It's attached to the cooler.
I had to do mine this year. The worst thing is getting the new one's thread to bite while the ATF is dribbling over your fingers , making it difficult to grip. It took me two goes, with one finger in the hole between times to stop the ATF from flowing out.
Of course you could take it as an opportunity to drain and change the ATF, and the filter too, but it may well still trickle out.
When I did it without draining it out, it took very little to top it up.
It'll only come on, if the sender is functioning properly, under extremely harsh conditions, like towing a heavy load, fast. up a mountain in 35 degree heat.